This project involves a comparative approach to the ultrastructure of spermatozoa, principally of invertebrates, with emphasis on gametes of turbellarian flatworms and molluscs. Such features as (1) patterns of microtubules in axonemes, (2) chromatin condensation during morphogenesis of spermatozoa, (3) cytoplasmic inclusions in sperm, and (4) particulate structures in and on sperm membranes are being examined, with the use of thin-sectioning, negative staining and freeze-etch techniques and electron microscopy. A growing body of evidence suggests that localized particulate modifications of membranes are found in regions where axonemes and mitochondria abut, in the form of "necklaces" and similar configurations demonstrated by the use of freeze-etch. We are finding a number of other such arrangements in spermatozoa.